A phase change memory (PCM) device is a non-volatile memory device employing a change in resistivity of a phase change material. The PCM device is also called phase-change random access memory (PRAM). Typically, a chalcogenide material capable of switching between an amorphous state and a crystalline state is employed in PCM devices.
Depending on the cooling rate from a liquid state, the chalcogenide material may form an amorphous chalcogenide glass or a chalcogenide crystal. The difference between the two states is physically characterized by the presence or absence of a long range order. Further, the crystalline and amorphous states of chalcogenide material have drastically different resistivity values. By manipulating the phase of the chalcogenide material, a binary data bit may be written into a PCM device. By detecting the phase of the chalcogenide material, typically in the form of a resistivity measurement, the binary data bit stored in the PCM device may be read. Many types of PCM devices employing these methods are known in the art.
A typical chalcogenide material used in PCM devices is a germanium, antimony and tellurium compound commonly called GST (Ge2Se2Te5). Along with oxygen, sulfur, selenium, and polonium, tellurium belongs to the chalcogen group, hence the name chalcogenide material. In a typical PCM device, a chalcogenide glass having a high resistivity value may be formed upon melting and rapid cooling of a chalcogenide material. Alternatively, a chalcogenide crystal having a low resistivity value may be formed by raising the temperature to a crystallization temperature, which is below the melting temperature, followed by a slow cooling of the chalcogenide material. The chalcogenide becomes liquid at a relatively high temperature, e.g., above 600° C.
Referring to FIG. 1, a prior art phase change memory (PCM) element structure comprises a stack of a bottom conductive plate 10, a phase change material layer 20, and a top conductive plate 30. By passing current through the PCM element structure, the phase change material in the phase change material layer 20 is heated to a temperature that can induce a phase change, i.e., either above the melting temperature or the crystallization temperature.
One of the challenges of present day PCM element structures is to generate sufficient heat to reach the melting temperature of the phase change material. While not every portion of the phase change material needs to melt to encode data in the PCM element, at least a portion of the phase change material capable of significantly affecting the overall resistance of the PCM element structure needs to reach the phase transition temperatures, i.e., the melting temperature and/or the crystallization temperature, in order for the phase change material to change its state between a crystalline structure and an amorphous structure. To induce such melting or recrystallization, a relatively large amount of current is typically required. However, application of such a large current requires a large transistor, and consequently a large semiconductor area, making it difficult to increase the density of PCM devices.
A method of reducing current demand on a PCM element structure by mixing a phase change material with an inactive dielectric material has been disclosed in the U.S. Pat. No. 5,825,046 to Czubatyj et al. FIG. 2 schematically represents the prior art structure by Czubatyj et al., in which a mixed phase change material layer 20′ comprises a phase change material 21 intermixed with the inactive dielectric material 26. The mixing of the phase change material 21 with the inactive dielectric material 26 decreases the cross-sectional area for the electrical current path between the bottom conductive plate 10 and the top conductive plate 30, thus increasing the current density within the mixed phase change material layer 20′. For a given level of current between the bottom conductive plate 10 and the top conductive plate 30, the mixed phase change material layer 20′ in FIG. 2 provides higher local temperature than the phase change material layer 20 in FIG. 1.
While Czubatyj et al. provides a structure that enhances the local temperature of a mixed phase change material layer, the mixing process is stochastic, i.e., statistical variations in the mixing process produces non-uniform mixing, resulting in significant variations in the resistance of the mixed phase change material layer.
Therefore, there exists a need for a phase change memory element structure capable of achieving phase transition temperatures in a phase change material layer consistently with less programming current and methods of manufacturing the same.
Further, there exists a need for a phase change memory element structure with a higher resistance in the phase change material layer, in which the resistance values have a tight distribution, and methods of manufacturing the same.